Holographic materials can be used for functional light management or for decoration. Decorative holographic materials make for appealing or eye catching displays. These materials are used for labels, boxboard lamination, flexible packaging, or other display or decorative applications. The appeal comes from light diffraction induced by a grating pattern either on the surface or buried in a lamination. Examples of holographic materials used for functional light management include diffraction gratings for monochrometers, retroreflective materials for enhanced road sign visibility, and holographic protective elements for security tags.
When incident light strikes the surface of a holographic material, it can be directed in one or more of five possible modes: transmission, absorption, direct reflection, diffraction and scatter. Transmission is the light that passes through the material without changing direction. Absorption is the light that is retained by the material. Direct reflection is the light that is returned at an equal and opposite angle to the incident light. Diffracted light is the light that is reflected or transmitted in discrete directions due to mutual interference mediated by periodic structures. Scatter is all light that is redirected irregularly including retroreflection, redirection from surface plasma polaritons, haze, surface roughness or imperfections, and Lambertian reflection. A description of the characteristics of surface relief diffraction gratings is found in M. G. Moharam et al, “Diffraction Characteristics of Photoresist Surface-relief Gratings” 23 Applied Optics 3214 (Sep. 15, 1984). A description of the components of reflection is found in Edward F. Kelley, et al. “The Three Components of Reflection” in Information Display, SID October 1998 pp. 24-29. Discussion of measurement of the color of objects is found in “Standard Practice for Obtaining Spectrometric Data for Object-Color Evaluation” ASTM Int'l pub. no. E1164-02. Discussion of methods for measuring retroreflective material is found in “Standard Test Method for Coefficient of Retroreflection of Retroreflective Sheeting Utilizing the Coplanar Geometry” ASTM Int'l pub. no. E810-01. All of the references stated herein are incorporated by reference.
Perceived visual quality of a holographic material has two components: Color intensity and color distinctness. Color intensity is the amount of diffracted light relative to the amount of incident light, or the diffraction efficiency. This is analogous to value in the hue, saturation, value (HSV) color model. In the case of surface relief patterns, color intensity can be theoretically modeled based on the depth, periodicity and shape of the surface relief pattern and the overall reflectivity of the surface. However, models are often very complex and can miss key features that may affect diffraction intensity such as surface plasma polaritons. Color distinctness is the sharpness of the colors or the color saturation. This is analogous to color saturation in the HSV color model. A holographic material can have good color intensity but have low perceived visual quality because the color distinctness is low. Light that is scattered by the sample reduces the distinctness of the colors. A holographic material will have a high perceived visual quality if it has high color intensity and high color distinctness.
Visual quality of a holographic material is an important property that is generally not measured or controlled. A common practice to determine the perceived visual quality is to perform a manual inspection with a trained observer. This subjective approach includes irregularities owing to factors such as lighting conditions, material orientation, and the observer's experience, mood or preferences. An automated, repeatable measurement process is desirable in order for manufacturers to produce consistent products.
An article written for the ACM SIGGRAPH 2003 Conference, Andrew Gardner et al., “Linear Light Source Reflectometry” describes a technique to estimate spatially varying reflectance properties of a surface based by taking a series of digital photographs of the surface while the surface is being illuminated with a single pass of a linear light source. This allows measurement of the diffuse color, and specular color of each point of the surface. A laser light source is also used to measure surface roughness. The article does not disclose a method or system for measuring perceived visual quality of a holographic material.
U.S. Pat. No. 5,615,005 discloses an instrument for grading gemstones. The device illuminates a gemstone with a plurality of light frequencies and measures an image of the gemstone illuminated at each frequency with a Charge Coupled Device (CCD) to obtain the spectral response of the gemstone.
An article in Pure Applied Optics by Lemelin et al. entitled “Holographic imaging of 3D objects on dichromated polymer systems”, vol. 5, 1996, pages 95-103, describes a method to measure the diffraction efficiency of a hologram using laser light.
Proceedings from Laser and Fiber-Optical Networks Modeling, September 2004, 6th International Conference in Kharkav, Ukraine by Litvinenko et al. entitled “The device of measurement of parameters of holograms” describes a method to measure the diffraction efficiency of a hologram
Patents in the field of holographic materials and/or diffraction grating include: U.S. Pat. Nos.: 7,157,135; 6,677,029; 6,120,710; 5,932,150; 5,756,183; 5,742,432; 5,737,125; 5,629,068 and 4,998,785, Japanese patents JP 09 196813 A and JP 04 145341 A, and U.S. published patent applications: US2006-0154050 and US2007-0091395. Also in the field are co-pending U.S. patent application Ser. No. 11/683,209: System and Method for Making Seamless Holograms, Optically Variable Devices and Embossing Substrates, filed Mar. 7, 2007 and Ser. No. 11/678,918, filed Feb. 26, 2007 Method for Optical Characterization and Evaluation of Optically Variable Devices and Media, filed Feb. 26, 2007. The prior art has not disclosed a method or instrument for characterizing visual quality of holographic materials. Having an instrument to characterize visual quality eliminates bias and provides acceptance criteria so that consistent reproducibility can be maintained.